1. Field of the Invention
The present invention generally relates to a magneto-optical storage device, and particularly to a magnetic field generator used for the magneto-optical storage device.
2. Description of the Related Art
In recent years, along with the improved processing abilities and processing speeds of personal computers, program capacities and data capacities of operating systems and application software programs have been increased. Under such situations, there have been strong demands toward enhancement of capacities and speeds as well as miniaturization and cost-reduction of external storage devices. As one kind of external storage devices, a magneto-optical disk drive has been developed and extensively used at present. The magneto-optical disk drive uses a magnetic body as a recording medium, and records and reproduces information by making use of light and magnetic fields.
Higher density recording has been required for the magneto-optical disk drive year by year, and to meet such a requirement, various methods for recording information by using the magneto-optical disk drive have been developed. Of these methods, a light modulation method has been generally used at present. The light modulation method records information on a magneto-optical recording medium by modulating a laser beam with recording data while applying an external magnetic field in a specific direction. In this light modulation method, a recording density of information is generally restricted by a size of a beam spot.
The magneto-optical disk drive includes means for generating a magnetic field necessary for recording/reproducing/erasing of information. The magnetic field generating means is represented by a magnetic field generator using a permanent magnet or an electric magnet. A prior art magnetic field generator using an electric magnet is shown in FIG. 1. The magnetic field generator is configured such that a center yoke 4 is fixed on a back yoke 2 formed of a steel plate. The center yoke 4 is formed of a cold-rolled steel plate specified in JIS (SPCC).
A coil 6 is inserted around the center yoke 4 in such a manner that the center yoke 4 is surrounded by winding of the coil 6. A top yoke 8 having an opening 10 is disposed over the coil 6. The center yoke 4 faces to the opening 10 having a specific width or gap. The top yoke 8 is also formed of a steel plate. In the case of a magneto-optical disk drive, the top yoke 8 is formed of part of a cartridge holder for accommodating a magneto-optical disk cartridge having been inserted in the disk drive.
When a specific current is applied to the coil 6, a magnetic field is generated from a surface of the center yoke 4. A magneto-optical disk (not shown) is rotatably supported while facing to the center yoke 4. An optical head is movably provided opposite to the center yoke 4 with respect to the magneto-optical disk, and the magneto-optical disk is irradiated with a light beam emitted from the optical head. A length of the center yoke 4 is substantially equal to a seek range of the optical head.
In the prior art magnetic field generator shown in FIG. 1, a width of the opening 10 of the top yoke 8 has been kept constant in the longitudinal direction thereof from the viewpoint of production ease. Accordingly, the shape effect of the opening 10 exerted on the magnetic field has not been examined at all. Further, as the material of the center yoke 4, there has been used a general cold-rolled steel plate being relatively low in relative magnetic permeability. This is one of causes of increasing the power consumption of the coil 6 for obtaining a necessary magnetic field.
FIG. 2 shows a magnetic field distribution of the prior art magnetic field generator in the longitudinal direction (radial direction of a magneto-optical disk), and FIG. 3 shows a magnetic field distribution of the prior art magnetic field generator in the lateral direction. In the graphs of FIGS. 2 and 3, the abscissa indicates a displacement from the center of the center yoke 4, and the ordinate indicates a magnetic field strength. In the magnetic field distribution in the longitudinal direction shown in FIG. 2, the non-uniformity of the magnetic field strength in a recording region is observed. In particular, the magnetic field strength becomes rapidly large at each end of the recording region. Accordingly, there is a possibility that a magnetic field having an excessively large strength is generated at each end of the recording region, and thereby the recorded information may be erroneously erased and the recording/reproducing/erasing conditions cannot be optimized at some recording positions.
FIG. 4 shows a magnetic field distribution in the height direction, wherein the abscissa indicates a distance from the center yoke to a magneto-optical disk and the ordinate indicates a magnetic field strength. As is apparent from this figure, a magnetic field in the height direction depending on a gap between the center yoke 4 and the magneto-optical disk becomes higher and thereby a magnetic field generation efficiency at the same coil current value becomes larger as the center yoke 4 comes closer to the magneto-optical disk.
In the prior art magnetic field generator, the gap between the center yoke 4 and the magneto-optical disk is set to 0.8 mm. However, in actual, an assembly variation of xc2x10.5 mm from a reference position (design position) RP is allowable. If the center yoke 4 comes excessively close to the magneto-optical disk, there may occur an inconvenience in which data be destroyed due to the contact therebetween. From this viewpoint, a nearest limit position NL is specified at about 0.3 mm.
In FIG. 4, a straight line LH designates a necessary lowest magnetic field strength, and C1 designates a magnetic field strength against an arbitrary current value. A curve C2 shown by a broken like designates a magnetic field strength curve after a current applied to the coil 6 is increased for obtaining a necessary magnetic field as shown by an arrow P. In this way, according to the prior art magneto-optical disk drive, to obtain a necessary magnetic field, it has been required to correct an assembly variation of the center yoke 4 by increasing a current applied to the coil 6.
An object of the present invention is to provide a magnetic field generator capable of increasing a magnetic field generation efficiency and realizing stable characteristics over the entire recording region.
Another object of the present invention is to provide a magneto-optical storage device capable of realizing stable characteristics over the entire recording regions.
In accordance with an aspect of the present invention, there is provided a magnetic field generator including: a coil; a center yoke disposed in a central portion of the coil; a top yoke having an opening from which an upper portion of the center yoke is exposed, the top yoke being disposed on an upper surface side of the coil; and a back yoke disposed on a back surface side of the coil; wherein the opening has a straight-line shaped central portion with its gap kept constant, and enlarged portions with their gaps becoming larger as going outwardly from the vicinities of both end portions of the center yoke.
Preferably, the center yoke is made from a high permeability material. More preferably, the magnetic field generator is provided with adjustably fixing means for adjustably fixing the back yoke to the top yoke. The adjustably fixing means may include a plate spring for biasing the back yoke in the direction in which the back yoke becomes apart from the top yoke, and fastening means for fastening the back yoke to the top yoke against a biasing force of the spring. By the provision of the adjustably fixing means, a gap between the center yoke and a surface of the magneto-optical recording medium can be adjusted at an optimum value, to thereby correct an assembly error.
In accordance with another aspect of the present invention, there is a provided magneto-optical storage device for storing information on a magneto-optical storage medium, including: a holder for accommodating the magneto-optical storage medium having been inserted in the magneto-optical storage device; an optical head for irradiating the magneto-optical storage medium with a light beam; and a magnetic field generator disposed on the holder; wherein the magnetic field generator includes: a coil; a center yoke disposed in a central portion of the coil; a top yoke having an opening from which an upper portion of the center yoke is exposed, the top yoke being disposed on an upper surface side of the coil; and a back yoke disposed on a back surface side of the coil; wherein the opening has a straight-line shaped central portion with its gap kept constant, and enlarged portions with their gaps becoming larger as going outwardly from the vicinities of both end portions of the center yoke.
In accordance with a further aspect of the present invention, there is provided a magneto-optical storage device for storing information on a magneto-optical storage medium, including: a holder for accommodating the magneto-optical storage medium having been inserted in the magneto-optical storage device; an optical head for irradiating the magneto-optical storage medium with a light beam; and a magnetic field generator disposed on the holder; wherein the magnetic field generator includes: a coil; a center yoke disposed in a central portion of the coil; a top yoke having an opening from which an upper portion of the center yoke is exposed, the top yoke being disposed on an upper surface side of the coil; a back yoke disposed on a back surface side of the coil; and adjustably fixing means for fixing the back yoke to the top yoke after adjustably setting a position of the back yoke to the top yoke.
The above and other objects, features and advantages of the present invention and the matter of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention.